Zoom lens thermal imager incorporating a non-pixellated detector

ABSTRACT

A thermal imaging device comprises a non-pixellated detector (3) which includes a component (12), such as a liquid crystal element, having optical properties which vary with temperature. An infra-red lens (1) forms a focused image of a thermal scene on the non-pixellated detector. A visible light source (17) is arranged to illuminate the non-pixelled detector such that visible light from the source is modulated by the image formed on the non-pixellated detector. A second detector (7), such as a television camera chip, detects the modulated visible light to give an output which is representative of the image of the thermal scene. A visible light zoom lens (5) is interposed in the optical path between the non-pixellated detector and the second detector so that variation of the focal length of the zoom lens varies the field of view of the thermal imaging device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to thermal imaging devices, and in particular tooptically-read thermal imaging devices.

2. Description of Related Art

Conventional thermal imaging devices include an infra-red sensitivedetector and one or more infra-red optical components which focus theradiation from a thermal scene on to the surface of the detector.Devices of this type are described in patent application Ser. No. GB2150387 and in our own patent application Ser. No. GB 2180361. Thematerial of the detector is chosen to have a highlytemperature-dependent optical property, such as birefringence or opticalrotation. This detector is also illuminated by a visible or near-visiblepolarized light source the output of which is reflected off a dichroicmirror on to the detector. The visible or near-visible light experiencesa modulation of polarisation, on passing through the detector, which isconverted to an intensity modulation by a quarter-wave plate and ananalyser. This intensity modulation conferred on the light passingthrough the detector thus corresponds to temperature variations inducedin the detector by the infra-red radiation. In conventional devices, themodulated light is then focused by lenses through a Fourier plane filteron to a television camera chip the output of which is read into a framestore.

In order to vary the field of view of the device, an infra-red zoom lensmust be used to focus the radiation from the thermal scene on to theliquid crystal. Infra-red lenses are expensive, and infra-red zoomlenses particularly so; so for any individual system, the range oflenses is usually limited.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a thermal imagingdevice in which the field of view may be varied, whilst avoiding the useof infra-red zoom lenses.

According to the present invention, a thermal imaging device comprises:a non-pixellated detector including a component the optical propertiesof which vary with temperature; means for forming a focused image of athermal scene on said non-pixellated detector; a visible light sourcearranged to illuminate the non-pixellated detector such that visiblelight from the source is modulated by the image formed on thenon-pixellated detector; a further detector effective to detect saidmodulated visible light, to give an output representative of the imageof the thermal scene; and a visible light zoom lens interposed in theoptical path between the non-pixellated detector and the furtherdetector such that variation of the focal length of the lens varies thefield of view of the thermal imaging device.

The component within the non-pixellated detector is suitably a liquidcrystal; the material of the liquid crystal may exhibittemperature-dependent optical properties, such as birefringence oroptical rotation.

Preferably the further detector which is effective to detect visiblelight is a pixellated detector such a a charge-coupled device of thekind used as a television camera chip. Alternatively a televisionvidicon may be used.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a thermal imaging device, and

FIG. 2 is a ray diagram.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In both figures, the rays shown incident on a liquid crystal detector 3are infra-red rays, whilst those emerging from the detector 3 are raysof visible light.

Referring first to FIG. 1, the device comprises means for forming afocused image of a thermal scene on to a detector 3, a visible lightsource 17 arranged to illuminate the detector 3, and a visible lightzoom lens 5 interposed between the detector 3 and a television camerachip 7.

The means for forming a focused image of the thermal scene includes achopper 9 which chops the infra-red radiation incident on an infra-redlens 1. This lens directs the radiation on to the full area of thedetector 3.

To illuminate the detector 3, a visible light from the off-axis source17 is converted into a parallel beam by a lens 19. The term `visiblelight` as used herein is intended to include light in the near infra-redand near ultra-violet parts of the spectrum which can be transmittedwithout significant attenuation by conventional optical lenses which aresubstantially transparent to light which is visible to the human eye. Apolarizer 21 is provided to polarise this beam in a direction parallelto the director of the liquid crystal which forms the thermo-opticcomponent of the detector 3 and exhibits optical rotation. The polarisedlight is then incident on a dichroic mirror 15 placed adjacent to a wall13 of a detector enclosure 11. The mirror 15 is so designed as totransmit infra-red radiation whilst reflecting radiation of otherwavelengths, and is angled so as to direct light from the source 17 onto the detector 3.

The detector 3 includes a liquid crystal cell 12 contained within atemperature-controlled enclosure 11, at least one wall 13 of which isdesigned to be transparent to infra-red radiation emitted from a fieldof view to be imaged. The liquid crystal mixture in the cell 12 isformed from two cholesteric liquid crystals; cholesteryl chloride whichhas a right handed structure and cholesteryl myristate which has a lefthanded structure. The alignment of the liquid crystal mixture is suchthat the pitch of the liquid crystal helix may adopt its natural valueat any particular temperature; in this way, the optical rotationexperienced by light passing through the liquid crystal cell will becontinuously variable as a function of temperature.

It is essential that the detector 3 should not be pixellated. In thiscase, the highest usable spatial frequency in the detector is determinedby the lateral heat spread which acts to degrade the high spatialfrequency variations in the detector temperature. Hence, the field ofview and the angular resolution of the system are not related by theproperties of the detector.

By means of the chopper 9, the detector 3 is alternately exposed to thethermal scene and a uniform temperature. The optical properties of theliquid crystal mixture in the cell 12 vary with temperature so that thepolarisation of the visible light incident on the detector 3 ismodulated on passing through the liquid crystal. This polarisationmodulation is then converted to an intensity modulation as the lightpasses through an analyser 23.

A visible light zoom lens 5 subsequently focuses the intensity-modulatedlight from the detector 3 on to a pixellated television camera chip 7,the output of which is fed into a frame store 27. The successive lightand dark frames formed in this way are subtracted from each other toprovide a video output.

FIG. 2 shows the zoom lens 5 which focuses the thermal image formed atthe detector 3 on to the television camera chip 7. The rays shown to theleft of the detector 3 are visible light rays and those shown to theright are beams of infra-red radiation which are focused on the fullarea of the detector at all times. The zoom lens 5 in a position Xfocuses the full field of view on to the camera chip 7, whereas in aposition X¹ a limited field of view is focused. In both cases the numberof pixels in the camera chip 7 determines the spatial resolution.Position X¹ is chosen so that the spatial resolution set by the camerachip matches that available in the detector 3.

The zoom lens 5 can take away any conventional form and, as isillustrated in FIG. 1, it is provided with a control signal input 6 bymeans of which the field of view of the zoom lens 5 is altered. Theeffect of altering the field of view of a zoom lens is to increase ordecrease its magnification. In the present case, the magnificationcannot usefully be reduced beyond that represented by the position X,but it can be increased until only a small central portion of thedetector 3 is within the field of view. It is under this lattercondition that the non-pixellated nature of the detector 3 is ofparticular importance, as any pixellated structure would significantlyimpose itself on the magnified field of view.

Thus the field of view of a thermal image can be altered over a verywide range using a conventional visible-light zoom lens.

If the component of the non-pixellated detector exhibits birefringence,the polarizer is chosen to polarise the beam of visible light in adirection which is at 45° to the director of the liquid crystal. In thiscase, a quarter-wave plate is interposed between the detector and theanalyser.

I claim:
 1. A thermal imaging device, comprising a non-pixellateddetector (3) including a component (12) the optical properties of whichvary with temperature; means (1) for forming a focused image of athermal scene on said non-pixellated detector; a visible-light source(17) arranged to illuminate the non-pixellated detector such thatvisible light from the source is modulated by the image formed on thenon-pixellated detector; a further detector (7) effective to detect saidmodulated visible light to give an output representative of the image ofthe thermal scene; and a visible light zoom lens (5) interposed in theoptical path between the non-pixellated detector and the furtherdetector such that variation of the focal length of the lens varies thefield of view of the thermal imaging device.
 2. A thermal imaging deviceas claimed in claim 1, in which the component (12) of the non-pixellateddetector (3) exhibits birefringence.
 3. A thermal imaging device asclaimed in claim 2, wherein the light source (17) is arranged off-axisand an inclined dichroic mirror (15) is arranged to deflect light fromthe source (17) onto the non-pixellated detector (3), the miror beingarranged in the path of radiation incident on said non-pixellateddetector.
 4. A thermal imaging device as claimed in claim 1, in whichthe component (12) of the non-pixellated detector (3) exhibits opticalrotation.
 5. A thermal imaging device as claimed in claim 4, wherein thelight source (17) is arranged off-axis and an inclined dichroic mirror(15) is arranged to deflect light from the source (17) onto thenon-pixellated detector (3), the mirror being arranged in the path ofradiation incident on said non-pixellated detector.
 6. A thermal imagingdevice as claimed in claim 1, in which component (12) of thenon-pixellated detector (3) is a liquid crystal.
 7. A thermal imagingdevice as claimed in claim 1, in which the further detector (7) is apixellated detector.
 8. A thermal imaging device as claimed in claim 7,in which the pixellated detector (7) is a charge-coupled device.
 9. Athermal imaging device as claimed in claim 8, in which the pixellateddetector (7) is a television camera chip.
 10. A thermal imaging deviceas claimed in claim 7, in which the pixellated detector (7) is atelevision camera chip.
 11. A thermal imaging device as claimed in claim7, wherein the light source (17) is arranged off-axis and an inclineddichroic mirror (15) is arranged to deflect light from the source (17)onto the non-pixellated detector (3), the mirror being arranged in thepath of radiation incident on said non-pixellated detector.
 12. Athermal device as claimed in claim 1, in which the further detector (7)is a television vidicon.
 13. A thermal imaging device as claimed inclaim 1, wherein the light source (17) is arranged off-axis and aninclined dichroic mirror (15) is arranged to deflect light from thesouce (17) on to the non-pixellated detector (3), the mirror beingarranged in the path of radiation incident on said non-pixellateddetector.
 14. A thermal imaging device as claimed in claim 1, whereinsaid non-pixellated detector (3) includes an analyser (23).